1. Field of the Invention
This invention relates to bubblers for supplying a vapor to a chemical process by introducing a carrier gas into a process chemical liquid to form a mixture of the vapor of the process chemical with the carrier gas.
2. Description of Related Art
U.S. Pat. No. 5,078,922 of Collins et al. shows a xe2x80x9cLiquid Source Bubbler.xe2x80x9d
U.S. Pat. No. 5,921,428 of Rodgers shows a xe2x80x9cSelf-Metering Reservoirxe2x80x9d.
An object of the invention is to provide a bubbler that suppresses splashing and the generation of aerosol droplets at high carrier gas flow rates, which exit the bubbler in the outlet flow mixture of the carrier gas and a chemical vapor, thus creating erratic variations in chemical mass transfer.
Another object of the invention is to provide the high flow rate of the carrier gas and the chemical vapor with anti-aerosol properties with a bubbler having a small internal volume.
A further object of the invention is to provide a high flow rate small volume bubbler, whose outlet concentration of chemical vapor to carrier gas is independent of the carrier gas flow rate.
Another object of the invention is to provide a high flow rate small volume bubbler, whose outlet concentration of chemical vapor to carrier gas is largely independent of the liquid level in the bubbler.
In accordance with the above objects, the invention provides an apparatus and method for generating a saturated mixture of a carrier gas and a chemical vapor devoid of chemical liquid droplets. The bubbler consists of a closed stainless steel bubbler container having a carrier gas inlet tube, a carrier gas/vapor outlet, a process chemical liquid fill inlet and a process chemical liquid drain outlet. The carrier gas inlet tube passes through the top of the bubbler container and into an enclosed plenum that distributes the carrier gas to a plurality of small generator tubes. The generator tubes extend from the bottom of the plenum down into the process chemical liquid in the bubbler container. The dimensions of the generator tubes are chosen such that at the maximum carrier gas flow rate the carrier gas stream exiting the generator tube into the liquid is a high velocity fully developed laminar flow comprising a cylindrical stream. Under these conditions the exiting cylindrical stream of carrier gas maintains a small diameter cylindrical shape in the process chemical liquid for a substantial distance from the outlet end of the generator tube. As the stream stretches farther away from the outlet end of the generator tube, the surface tension at the carrier gas/process chemical liquid interface acts to pinch off the cylindrical stream of carrier gas into a series of small bubbles whose diameter is primarily a function of the diameter of the cylindrical stream of carrier gas and the surface tension. The bubble diameter is almost independent of flow rate. The series of small bubbles rises up through the process chemical liquid and quickly becomes fully saturated with chemical vapor due to their large surface-area-to-volume ratio. A further benefit of maintaining small bubble size is that the rate of bubble ascent is limited, thus increasing contact time with the process chemical liquid while minimizing splashing and the formation of aerosol droplets of liquid when the bubble breaks the surface of the process chemical liquid. The carrier gas vapor outlet port extends through the top of the bubbler container and is located behind the plenum such that the plenum acts as a baffle to shield the carrier gas vapor outlet port from the surface of the process chemical liquid as a further means of preventing any liquid from entering the outlet stream.
Chemical liquid level measurement means measure the chemical liquid level inside the bubbler container to provide for chemical liquid level alarm conditions and for automatic filling. A piezo-ceramic transducer is bonded to the outside surface of the bottom of the bubbler container in an area aside from the location of the generator tubes. An electrical signal is applied to the piezo-ceramic transducer that generates an elastic wave that propagates through the bottom of the stainless steel bubbler container and into the process chemical liquid. The acoustical wave propagates through the process chemical liquid and is almost totally reflected at the surface of the process chemical liquid due to the mismatch in acoustical impedance between a liquid and a gas. The reflected acoustical wave propagates back through the liquid and the bottom of the bubbler container and is received by the piezo-ceramic transducer, thereby producing an electrical signal, which is detected and processed to determine the time delay between the transmitted and received signals. The height of the liquid above the piezo-ceramic transducer is calculated as a function of the measured time delay and the known speed of sound in the liquid. Because the speed of sound in a liquid is almost independent of the chemical composition of the liquid, a generic speed of sound of 1,300 meters per second can be used and still maintain a liquid level measurement accuracy of xc2x110%.
The column of process chemical liquid above the piezo-ceramic transducer is partially isolated from the bulk of the process chemical liquid volume by a stainless steel baffle attached to the inside wall of the bubbler container. The baffle keeps the process chemical liquid surface above the piezo-ceramic transducer relatively smooth, further enhancing the accuracy of the time delay measurement. Small gaps at the top and bottom of the baffle connect the volume enclosed by the baffle with the rest of the volume of the bubbler container, thus allowing the height Hxe2x80x2 of the process chemical liquid level in the volume enclosed by the baffle to remain in equilibrium with the height H of the process chemical liquid level in the main volume of the bubbler container.
Temperature control means allow bubbler operation above ambient temperature to increase the outlet concentration of chemical vapor in the carrier gas. These include a molded silicon-rubber insulating jacket that encapsulates the bubbler container and inlet and outlet fittings, heating elements bonded to the exterior surfaces of the bubbler container, a temperature measurement means and a temperature feedback control means.